Over time the surgical technique for total hip replacements has evolved. Incision length has been reduced over time as surgeons become more comfortable operating with limited visibility. The location of the incision has also been changing as surgeons have developed and implemented different approaches to the joint. These two factors have increased the challenges of implanting acetabular implants in the correct orientation as the acetabular tools impinge on bone or soft tissue around the perimeter of the incision.
The present invention relates to a prosthetic acetabular cup inserter and impactor, for use particularly, but not exclusively, in minimal invasive surgery (MIS) with a short incision and/or with an anterior approach.
Prosthetic acetabular cup inserter and impactors are used to implant prosthetic acetabular cups into the cavity of a patient's hip, and generally comprise an elongate straight or curved body with a cup engaging head at a first end thereof, and a handle and impaction plate or anvil at a second end thereof. The surgeon releasably fits a cup implant to the cup engaging head, and then positions the cup inside the patient's hip. He then applies a hammering force to the impaction anvil to secure the cup in place, before releasing the cup from the cup engaging head.
In Minimally Invasive Surgery (MIS) or in an anterior approach, it is common to use an inserter and impactor with a curved body, which is shaped to minimize impingement to the patient's soft tissue when it is inserted. In many cases the cups have irregular shapes and/or holes which have to be aligned inside the patient at a particular angle. With such constructions it is important to axially align the cup correctly in relation to the body of the inserter and impactor prior to insertion, so it is properly aligned with the patient's hip when the curved body is inserted as desired.
Acetabular implants normally consist of a shell or cup and a modular insert that fits within the shell and acts as a bearing surface for the femoral head. Such shells and inserts are shown in U.S. Pat. No. 6,475,243, the disclosure of which is incorporated herein by reference. While modular shells and inserts are preferred for a number of reasons, there is an application where shells and inserts are combined preoperatively in a monoblock construction.
Acetabular instruments normally consist of a series of reamers, a reamer handle, a shell positioner/impactor and an insert positioner/impactor. In addition, alignment guides are often attached to the reamer handle and shell positioner/impactor in order to facilitate alignment. A typical reamer is shown in U.S. Pat. Nos. 4,023,572 and 5,658,290.
Shells are implanted into an acetabulum after the acetabulum has been prepared to receive the shell usually through the use of a series of reamers increasing in size. The shells are aligned in the acetabulum according to two angles: abduction and anteversion. The combination of these two angles creates the axis that the shell should be aligned and impacted on.
Traditionally acetabular reamer handles and shell and insert positioner/impactors had straight shafts. In some surgeries the size or location of the incision results in the shaft of these instruments impinging on the side of the incision before the preferred abduction/anteversion axis is achieved. In these cases the surgeon has to force the soft tissue or bone out of the way, increase the length of the incision, or accept the abduction/anteversion angle that can be achieved. None of these options are preferred.
One method for avoiding impingement between these acetabular instruments and the incision is to create “inline” or offset curved acetabular instruments. “Inline” refers to an acetabular instrument that has a curved section between the two ends of the instrument that lie on the same axis. Typically, with regard to reamers, the first end includes a hex connection for connection to a rotary power service (drill) and a second end which has a holder for an acetabular instrument such as a reamer or the acetabular shell or a connector for the shell. “Offset” refers to an acetabular instrument that has a curved section between the two ends of the instrument that lie on different axis. The curved section should begin as quickly as possible after the attachment to the reamer, shell impactor or inserter in order to minimize the impingement.
An inline curved acetabular instrument is preferred for a number of reasons. Typically, the surgeon is used to operating with inline straight instruments. By maintaining the inline aspect of the design, the ergonomics of the instrument remain the same and the surgeon learning curved is reduced. In addition an inline instrument allows for all forces to be projected in line or parallel to the correct axis. Having the instrument inline is very important when used as an inserter/impactor to reduce the moments that result. Finally, an inline instrument allows for alignment guides to be indexed around the axis of the instrument without changing its position in relation to the implant. In-line or offset reamers and impactors are shown in U.S. patent publications 2003/0050645, 2003/0229356, 2004/0153063, 2004/0087958, 2005/0038443 and 2005/0216022.
Shells are typically connected to positioner/impactors by threading the two together with torque. This is accomplished through the use of a straight shaft with a handle that the surgeon grips on one end and a threaded fitting that connects to the shell on the other end. The torque is transmitted from the surgeon to the handle to the shaft to the threaded fitting to the shell. Typically the acetabular cup shell is metal with a hemi-spherical shape. A threaded central hole is located at the pole of the hemisphere. The threaded tip of the inserter/impactor engages this threaded bore. The axial force is transmitted from the surgeon to the mallet to the handle, to the shaft, to the threaded connector and to the pole area of the metal shell.
With an inline or offset curved acetabular instrument the axial load needs to be transmitted around a curve. In addition, the curved body that transmits the axial load cannot also transmit the torque. This is because the curved body would impinge on the incision if one tried to rotate it through a full rotation.
The curved body is preferably machined from a solid block referred to as a monoblock, to produce the curved body rather than using a bent tube. This allows the formation of an I-BEAM for increased stiffness and therefore precision because of I-BEAM construction.